Virus structures haave been analyzed in the cases of (i) Tobacco mosaic virus, where the average radial density distributions of both the virion and RNA-free polymers of TMV coat protein have been derived from dark-field electron images of unstained specimens. Both particles have outer diameters of 17.4nm, central holes 3.5nm in diameter, and the virion contains an additional density peak at a radius of 4nm contributed by the packaged RNA molecule. (ii) Vesicular Stomatitis Virus has also been analyzed by this technique. Unstained, its full diameter is about 84nm, and distinct radial features have been identified with the external glycoprotein spikes, the membrane envelope, and the coiled nucleocapsid containing the RNA located at a radius of 16nm. (iii) Conformational transformations of the surface lattice of Bacteriophage T4 capsid have been characterized by combining differential scanning calorimetry with electron microscopy. In all states studied, the inter- and intrasubunit bonds are found to be comparably stable in respect of thermal perturbation, as the polyheads denature as single cooperative units at temperatures characteristic for each of the different conformational states. Two minor endotherms were also observed which are inferred to represent, in one case, denaturation of the N-terminal portions of the gp23 molecules, and in the second case, denaturation in situ of hoc proteins in the mature capsid structure. In addition, we have studied the membranes of coated vesicles purified from rat liver and found them to contain cholesterol in amounts typical for hepatocyte plasma membranes, but to be morphologically susceptible to filipin, a cytochemical probe for cholesterol, only when their clathrin coats are removed. In consequence, the attribution of a cholesterol-filtering capacity to endocytic coated pits is no longer tenable. Finally, we refined our general structural model for Intermediate Filaments, and initiated a project to determine the helical organization of bacterial pili.